1. Field of the Invention
This invention generally relates to a linear motion rolling contact guide unit, and, in particular, to a finite linear motion rolling contact guide unit including a retainer for retaining a plurality of rolling members interposed between a rail and a slider.
2. Description of the Prior Art
A linear motion rolling contact guide unit is well known in the art and it generally includes a rail or base, a slider slidably mounted on the rail and a plurality of rolling members interposed between the rail and the slider. There are basically two types of such guide units. That is, one type of such guide units is of the endless linear motion type in which the slider may move along the rail as long as the rail exits; whereas, the other type is of the finite linear motion type in which the slider may move relative to and along the rail over a limited distance. In the case of the finite linear motion type, use is typically made of a retainer for retaining the rolling members in position between the rail and the slider.
One typical prior art finite linear motion guide unit is schematically shown in perspective view in FIG. 20. As shown, the illustrated finite linear motion guide unit includes a rail 22, which is elongated in shape and extends over a desired distance, and a slider or table 21 which is substantially equal in length in the longitudinal direction to the rail 22. The rail 22 is generally rectangular in shape in the illustrated structure and provided with a pair of left and right inner guide grooves 33 on its opposite side surfaces. The slider 21 is generally U-shaped in cross section and slidably mounted on the rail 22 in a straddling manner. The slider 21 is provided with a pair of left and right outer guide grooves 34 each located spaced apart from and facing opposite to a corresponding one of the pair of inner guide grooves 33. Thus, a guide channel is defined between each associated pair of inner and outer guide grooves 33 and 34, and a plurality of rolling members or rollers 23 in the illustrated example are provided in the guide channel. As a result, the rollers 23 are partly received in both of the associated inner and outer guide grooves 33 and 34 so that a rolling contact is provided between the rail 22 and the slider 21. In the illustrated example, the rollers 23 are arranged in the so-called crossed arrangement in which any two adjacent rollers 23 are arranged with their rotating axes directed perpendicular to each other when viewed into the direction of advancement.
As shown in FIG. 20, also provided is a retainer 24 in the form of an elongated plate for retaining the rollers 23 in position in the guide channel. In this structure, because of the provision of the retainer plate 24, the rollers 23 can be maintained at desired positions even if a relative motion is produced between the rail 22 and the slider 21 so that a uniform distribution of load may be maintained.
The outer guide groove 34 is defined in an elongated guide member 32 which is fixedly attached to each side wall section of the slider 21 by means of bolts 30 which are threaded into mounting holes 31. An end plate 26 is fixedly attached to each end of the rail 22 by means of screws 27 for blocking the inner guide groove 33 at each end so that the rollers 23 are prevented from slipping away.
In the linear motion guide unit of the above-described structure, a relative motion between the rail 22 and the slider 21 is rather limited. That is, there is a limit for the slider 21 to move to the right or to the left relative to the rail 22. Because of this limited range of relative movement between the slider 21 and the rail 22, there is a limitation in providing mounting holes 28 for use in mounting the rail 22 on a desired object, such as a frame of a machine, in the rail 22. In particular, in the case of the linear motion guide unit of the type shown in FIG. 20, such mounting holes 28 are only allowed to be provided at each end portion of the rail and not at the central portion because the central portion is not exposed even if the slider 21 is moved either to the leftmost or rightmost position relative to the rail 22.
This can be a significant disadvantage since even if the rail 22 is high in precision, an error in positioning or distortion can be created when the rail 1 is fixedly mounted on a desired object, such as a frame of a machine, by means of bolts or the like. That is, there is a chance that the linearity or planarity may be degraded when mounted on a desired object, for example, by bolt tightening. Besides, since the mounting holes 28 are provided only at each end portion of the rail 22, the manner of mounting the rail 22 is rather limited, which, in turn, could limit the scope of application of the guide unit shown in FIG. 20.